Childhood herpes simplex encephalitis (HSE) is a life-threatening complication of primary infection by herpes simplex virus 1 (HSV-1), a common virus that is innocuous in most children. HSE is the most common sporadic viral encephalitis in Western countries and acyclovir-treated survivors often suffer from severe neurological sequels. The pathogenesis of HSE remained unclear until we recently showed that the disease may result from monogenic traits specifically impairing immunity to HSV-1 in the central nervous system (CNS), at least in some children. So far, we have described two children who presented with HSE because of autosomal recessive UNC-93B deficiency and two others who presented with HSE due to autosomal dominant TLR3 deficiency. By inference from experiments conducted using fibroblasts, we found that the pathogenesis of HSE in these patients involves impaired interferon (IFN)-alpha/beta and -lambda production upon stimulation of UNC-93B- dependent TLR3 by dsRNA viral intermediates in the CNS, which results in increased viral replication and enhanced cell death. Only four of the 92 patients tested carried either genetic defect. We now hypothesize that pediatric HSE in other children results from other single-gene inborn errors of immunity. The principal objective of the present application is to identify novel HSE-causing genes by following a hypothesis-free and hypothesis-generating, genome-wide screening approach focused on consanguineous families. In this approach, we will search for and characterize HSE-predisposing genes by genome-wide linkage and homozygosity mapping, taking advantage of 19 consanguineous kindreds. We have already obtained preliminary evidence that there are at least two homozygous regions linked to HSE, in which new HSE- predisposing genes HSEL1 and HSEL2 are likely to be identified. Our project is highly innovative, clearly feasible, and supported by promising preliminary evidence. From a basic biological standpoint, this research will provide unique insight into the mechanisms of immunity to HSV-1 in the CNS. The elucidation of the molecular genetic basis of HSE will also illuminate the pathogenesis of a devastating pediatric illness, making molecular diagnoses available for patients and genetic counseling possible for families. This new information will pave the way for new therapeutic avenues, such as the use of IFN-alpha in addition to acyclovir. Finally, the genetic dissection of HSE will provide proof-of-principle that sporadic, life-threatening infectious diseases in otherwise healthy children may result from collections of single-gene inborn errors of immunity. This paradigm shift for primary immunodeficiencies will create novel avenues for the investigation of infectious diseases.